Arteriosclerosis, a chronic disease characterized by abnormal thickening and hardening of the arterial walls, is a leading cause of death in the United States. Arteriosclerosis begins when fatty streaks form within the arteries along the arterial walls. Over the years, these streaks enlarge, form connective tissue and calcium, and often cause blockage of blood flow through the arteries. If these blockages, called plaques, occur in the coronary arteries, they may cause a heart attack. Blockages in the arteries of the head and neck may cause strokes and blockages in the arteries of the legs may cause gangrene.
The most commonly used remedy for arteriosclerosis is bypass surgery, a major operation in which veins are taken from the patient's limbs and grafted onto the diseased artery to carry blood around the obstruction in the artery. Bypass surgery is an expense and dangerous procedure which is often less than 100% effective.
An alternative to bypass surgery is a recently developed technique called balloon angioplasty. Balloon angioplasty requires that a catheter be inserted into an artery of the arm or leg and pushed through the arterial passage until it reaches the partially blocked area of the artery. A tiny balloon on the tip of the catheter is then inflated to deform the plaque deposits, thus widening the arterial channel and facilitating the flow of blood through the vessel. Balloon angioplasty is not an appropriate treatment for every case of arteriosclerosis. In the cases not treatable by balloon angioplasty techniques, the arteries are either too clogged to permit insertion of the catheter into the region of the blockage, or the calcified fat deposits are too hard to be deformed. Even in the cases where balloon angioplasty is applicable, the remedy may not last for longer than a year before the blockage reoccurs.
A promising new development for the treatment of arteriosclerosis is a procedure called laser angioplasty. This procedure utilizes a catheter which is inserted into the artery and advanced to the site of the obstruction. An optical fiber is inserted through the catheter and positioned at the obstruction. The optical fiber is attached on one end to a laser. The laser energy which is guided through the fiber exits at the distal end of the fiber and is absorbed by the obstruction. Through a variety of mechanisms, depending in part on the wavelength of the laser energy, the obstruction is destroyed by the absorption of the laser energy and thus removed from the artery. The major advantage of laser angioplasty over balloon angioplasty is that the obstruction is removed from the artery. Furthermore, in contrast to balloon angioplasty, laser angioplasty may be used where there is a complete blockage of the artery.
Presently, three types of lasers, CO.sub.2, Nd:YAG, and argon ion, are commonly used in medical applications. The choice as to which laser to use for a particular application depends on a number of factors, as the effect of laser light on tissue varies with the wavelength, intensity, and mode of delivery of the laser light. In general, the mode of delivery for a laser is either continuous output or output in the form of a series of pulses.
CO.sub.2 lasers emit light in the far infrared, wavelength of 10,600 nm, and are typically used in the continuous mode to ablate tissue. Since the laser energy is readily absorbed by the tissue water, it penetrates the tissue to a depth of only 50 .mu.m to 100 .mu.m. If the absorbed far infrared light energy is sufficiently intense, the water component within the cell may vaporize into steam and heat the surrounding organic material. This photothermal ablation of tissue is the common mechanism of tissue vaporization by conventional medical lasers.
Nd:YAG lasers emit light at wavelengths of 1060 nm and 1318 nm. Operated in the continuous mode, Nd:YAG laser energy penetrates tissue more deeply than CO.sub.2 laser energy and is absorbed largely by proteins. As larger volumes of tissue are heated with the same amount of energy, Nd:YAG laser energy tends to produce tissue necrosis and coagulation before it produces significant photothermal ablation.
Argon gas lasers emit a blue-green light at wavelengths of 488 nm and 514.5 nm. The argon laser energy is most strongly absorbed by pigmented material, particularly hemoglobin and myoglobin. Therefore, its penetration depth varies with the color of the target tissue. Argon gas laser energy is commonly delivered in the continuous mode and modifies or destroys tissue by the photothermal ablation mechanism described previously in connection with CO.sub.2 laser energy.
The CO.sub.2, Nd:YAG, and Argon lasers have two features which are significant limitations for application to laser angioplasty. First, all of these lasers produce their effect by thermal effects which can cause thermal injury to surrounding healthy tissue. Second, none of these lasers are capable of ablating densely calcified tissue. Additionally, there are no sufficiently flexible fiber optics which are capable of transmitting CO.sub.2 laser energy, and the fiber optics for transmission of the argon and Nd:YAG laser energy tend to melt when intravascular debris accumulates on the tip of the fiber.
The limitations of the CO.sub.2, Nd:YAG, and argon lasers for applications to laser angioplasty lead to a search for a laser having characteristics more suitable for the procedure. It was found that lasers emitting light in the ultraviolet (UV) region of the spectrum possess three important characteristics which are potentially advantageous for medical applications and particularly for laser angioplasty. First, UV light is intensely absorbed at the surface of living tissue. Second, UV light produces little or no thermal injury to living tissue and finally, it ablates densely calcified material. In contrast to the ablation produced by CO.sub.2, Nd:YAG and argon lasers, ablation produced by UV laser energy has been found to: (1) produce an incision that conforms precisely to the laser beam configuration with no evidence of carbonization of surrounding tissue, (2) produce minimal thermal injury to surrounding tissue, and (3) preserve the tissue architecture. In addition, unlike the CO.sub.2, Nd:YAG and argon lasers, ultraviolet lasers can cut through bone, opening possibilities for precise facial reconstruction. Additionally, ultraviolet lasers open possibilities for advanced ophthalmologic procedures such as radial keratotomy and delicate neurosurgical procedures.
The superior performance of ultraviolet photons is believed to be due in part to the mechanism of tissue interaction. Specifically, ultraviolet photons carry enough energy to break chemical bonds. As a result, the photons chemically dissociate the molecules comprising the tissue. The tissue is vaporized because the fragments of the molecular dissociation occupy a larger volume than the original material. This process, known as photochemical ablation, removes plaque from blood vessels with minimal generation of heat, and the heat that is produced is carried away by the ejected fragments. Greater precision is thought to occur with the photochemical ablation process because tissue is no longer destroyed by thermal energy. Alternatively, the exquisite precision observed with ultraviolet laser energy may result from the application of the laser energy in pulses having very short durations (10-300 nsec) and at frequencies ranging from ten hertz to several hundred hertz. Thus, the off time between pulses allows the tissue to cool before additional energy is deposited at the site.
Ultraviolet laser energy suitable for medical applications can be produced by a class of lasers called excimer lasers. The wavelength produced by the excimer lasers depends on the type of gas used for the lasing medium. For example, argon fluoride (ArF), krypton fluoride (KrF), xenon chloride (XeCl), and xenon fluoride (XeF) excimer lasers generate ultraviolet light having wavelengths of 194 nm, 248 nm, 308 nm, and 355 nm, respectively. All of these wavelengths are in the ultraviolet and possess advantageous properties for plaque removal. Thus, excimer lasers are currently the leading candidates for use in laser angioplasty applications.
Notwithstanding the clear advantages of ultraviolet laser energy for angioplasty applications, the technique has not been met with widespread acceptance for laser angioplasty because of two significant hardware limitations. First, the high peak power, defined as the pulse energy divided by the pulse width, and absorptive characteristics of the excimer laser energy within an optical fiber are difficult to control. This may result in damage to the fiber transmission system, leading to a short useful life for the fiber. Second, the laser gases are highly toxic and have, in the past, not been generally acceptable for use in the environment of a hospital or operating room.
Many of the deficiencies found in presently used CO.sub.2, Nd:YAG, and argon laser based angioplasty systems could be overcome by use of the ultraviolet photon energy delivered by excimer lasers. Unfortunately, even though the advantages of excimer lasers for laser angioplasty are widely recognized, prior systems have been restricted to use in experimental settings because of limited lifetimes of the lasers and optical transport systems and the need to isolate the toxic excimer laser gases from the surrounding environment.
Accordingly, there is a need for a laser angioplasty system which utilizes excimer lasers and is capable of operating for extended time periods without interruptions. Additionally, the system should have provision for isolating the excimer laser medium gases from the hospital and operating room environments.